Gamma-secretase catalyzes the cleavage of multiple substrates, several of which are known to play a role in the function of a normal immune system. Our hypothesis is that gamma-secretase inhibition by LY411,575 (hereafter referred to as LY) will have immunosuppressive/anti-inflammatory effects both in vitro and in vivo. In our preliminary data we show that inhibition of gamma-secretase has several profound effects on immune function. In particular, we provide evidence that blockade of gamma-secretase results in inhibition of T cell proliferation, interferon-gamma (IFNgamma) secretion, NF-KB activation, antibody production, and differentiation of naive CD4+ T cells into Th1 cells. While in many instances we have convincing data suggesting that gamma-secretase exerts its effects on the immune system through the activation of Notch signaling, other targets of gamma-secretase require consideration. In this application, we present four specific aims designed both to elucidate the role of gamma-secretase in the functioning of a normal immune system and to identify the physiological target(s) of this enzyme in normal immune function. Additionally, because we have strong evidence linking inhibition of gamma-secretase to decreased severity of experimental autoimmune encephalomyelitis (EAE), a well-documented Th1-mediated disease, we will extend our studies of normal immune responses to an examination of the role of gamma-secretase in the initiation and exacerbation of EAE. We also will explore the possibility that LY induces a population of regulatory T cells. Finally, we have evidence that inhibition of gamma-secretase results in significant inhibition of NF-KB activity, a protein with prominent roles in both tumor development as well as inflammation. In our last aim, we describe experiments designed to determine the mechanisms by which gamma-secretase may regulate downstream NF-KB activity. The goal of this proposal is to establish the role played by gamma-secretase in normal immune function. These studies will provide valuable data that both inform us of the efficacy of the use of gamma-secretase inhibitors as a potential therapeutic agent in inflammation, as well alert us to any undesirable effects of this inhibitor on normal immune function.